In process and automation technology, physical parameters, such as e.g. mass flow rate, density and/or viscosity, of a medium flowing in a pipeline are often measured using inline measuring devices, which, by means of a vibration-type measurement pickup flowed-through by the medium and a measuring and operating circuit connected thereto, bring about, in the medium, reaction forces, such as e.g. Coriolis forces corresponding with the mass flow rate, inertial forces corresponding with the density of the medium and/or frictional forces corresponding with the viscosity of the medium, etc. and produce, derived from these, a measurement signal representing the current mass flow rate, the current viscosity and/or the current density of the medium.
Such measurement pickups, especially such as are embodied as Coriolis mass flow meters or Coriolis mass flow/density pickups, are described extensively and in detail e.g. in WO-A 04/099735, WO-A 04/038341, WO-A 03/076879, WO-A 03/027616, WO-A 03/021202, WO-A 01/33174, WO-A 00/571/41, WO-A 98/07009, U.S. Pat. Nos. 6,880,410, 6,851,323, 6,807,866, 6,711,958, 6,666,098, 6,308,580, 6,092,429, 5,796,011, 6,006,609, 5,602,345, 5,301,557, 4,876,898, 4,793,191, EP-A 553 939, EP-A 1 001 254, EP-A 12 48 084, EP-A 1 448 956, or EP-A 1 421 349. For conveying the, at least at times, flowing medium, the measurement pickups include at least one pickup tube held appropriately oscillatably in a, most often, thicker, especially tubular and/or beam-like, support cylinder, or in a support frame. In addition, these measurement pickups have a second pickup tube, mechanically coupled with the first pickup tube at least by means of two, especially, however, four, coupling elements (also called node plates, or couplers) and likewise vibrating, at least at times, wherein at last the first pickup tube is embodied as a first measuring tube communicating with the pipeline and serving to convey the medium to be measured. For producing the aforementioned reaction forces, the two pickup tubes are caused to vibrate during operation, driven by a, most often, electrodynamic exciter mechanism, with the two pickup tubes executing, at least at times, bending oscillations about an imaginary oscillation axis essentially parallel to a longitudinal axis of the measurement pickup. For registering vibrations, especially inlet-end and outlet-end vibrations, of the pickup tube and for producing at least one oscillation measurement signal representing the vibrations, such measurement pickups have, additionally, in each case, a sensor arrangement reacting to movements, and, to such extent, also to mechanical oscillations, of the pickup tube.
During operation, the measurement-pickup inner oscillation system, formed by the at least one pickup tube embodied as measuring tube, the medium conveyed at least instantaneously therein, and, at least partly, by the exciter mechanism and the sensor arrangement, is excited by means of the electromechanical exciter mechanism to oscillate mechanically, at least at times, in a wanted oscillation mode at at least one, dominating, wanted oscillation frequency. These oscillations in the so-called wanted oscillation mode are, most often, and especially in the case of use of the measurement pickup as a Coriolis mass flow- and/or density-meter, developed, at least partially, as lateral oscillations. Selected as the wanted oscillation frequency is, in such case, usually a natural, instantaneous resonance frequency of the inner oscillation system, which, in turn, depends both on the size, the shape and the material of the pickup tube and also on an instantaneous density of the medium; under the right circumstances, the wanted oscillation frequency can also be influenced significantly by an instantaneous viscosity of the medium. As a result of fluctuating density of the medium to be measured and/or as a result of medium change produced during operation, the wanted oscillation frequency is naturally changeable during operation of the measurement pickup, at least within a calibrated and, to such extent, predetermined, wanted frequency band, which has, correspondingly, a predetermined lower limit frequency and a predetermined upper limit frequency.
The inner oscillation system of the measurement pickup formed in common by the least one pickup tube, the exciter mechanism and the sensor arrangement is, additionally, usually housed by a pickup housing including, as an integral component thereof, the support frame, or support cylinder, as the case may be. The pickup housing is mechanically coupled with the pipeline via an inlet end and an outlet end. Pickup housings appropriately suited for vibration-type measurement pickups are described, for example, in WO-A 03/076879, WO-A 03/021202, WO-A 01/65213, WO-A 00/57141, U.S. Pat. Nos. 6,776,052, 6,711,958, 6,044,715, 5,301,557, and EP-A 1 001 254. Especially in the case of measurement pickups with bent pickup tubes, the pickup housing has a housing cap connected with the support frame, especially welded therewith. The housing cap surrounds the pickup tube, at least partially.
The measurement pickup housing serves, besides holding the at least one measuring tube, especially also for protecting the measuring tube, the exciter mechanism and the sensor arrangement, as well as other internal components, from external, environmental influences, such as e.g. dust or water spray. Examples of corresponding housing caps for a vibration-type measurement pickup for housing at least one bent tube segment, which, as part of a fluid-conveying measuring tube, vibrates during operation of the measurement pickup, are described e.g. in WO-A 03/021202, WO-A 03/021203, WO-A 00/57141, U.S. Pat. No. 5,301,557, and EP-A 1 001 254.
Users frequently demand of such housings that, in the case of an unsealed or bursting measuring tube, they withstand, leak-free, at least for a specified period of time, the static internal pressure, which then, most often, lies distinctly above the external pressure; compare, in this connection, also WO-A 00/57 141, U.S. Pat. Nos. 6,044,715, 5,301,557, or EP-A 1 001 254. At least for applications with toxic or easily ignitable fluids, the measurement pickup housing must also, in certain circumstances, be able to fulfill the requirements for a safety container. A problem associated therewith is, however, especially for applications with media under high static pressure of over 100 bar, that, after the measuring tube has become unsealed and, therefore, the measurement pickup housing is, under the right circumstances, loaded with an increased internal pressure, an explosion of the measurement pickup housing and/or an electronics housing appropriately affixed to the measurement pickup housing for the measuring device electronics can unexpectedly occur, which, while delayed, is nevertheless just as devastating in effect. This can especially occur, when the pipeline conveying the medium is loaded with unpredictably high pressures and/or with a series of pressure shocks of unpredictably high frequency and/or repetition rate. Beyond this, the measuring tube and measurement pickup housing can also fail due to material flaws and/or fatigue, even after long periods of operation, at pressure values which are really quite within specifications.
On the other hand, it is oftentimes not possible, especially in the case of environmentally endangering media, for example highly toxic and/or highly explosive substances, to use otherwise appropriate safety outlets, such as e.g. burst disks and/or excess pressure valves, for reducing possible excess pressures in the measurement pickup, since a contamination of the environment with the medium must, most often, be prevented with certainty.